Cat’s Back, western Herefordshire – the seventh in a series about top Earth Heritage sites in Herefordshire and Worcestershire.
Mike Brooks writes: this is a great place for a walk in dramatic upland scenery (pick good weather) to explore Old Red Sandstone rocks and glacially sculpted landscapes.
On the western extremity of Herefordshire is a razor sharp ridge, known as the Cat’s Back, climbing steeply from the Olchon Valley to the summit of Black Hill on the eastern edge of the Black Mountains.
Cat’s Back is a great place to see geological activity at two very different times, firstly, the hills are built from 400 million year old Devonian sedimentary rocks and, secondly, the relatively recent action of ice in sculpting the landscape.
As you climb the Cat’s Back, starting from the Black Hill car park (Grid Ref. SO288328), the ground is a red-brown colour and often muddy. as you crossing the red mudstones and sandstones of the Freshwater West Formation (formerly known as the St. Maughans Formation). These rocks were deposited as the flood deposits of extensive southward flowing rivers. Some 400 million years ago what would become the England of today lay around 25 degrees south of the equator on the southern flanks of a great continent from which rivers carried sediment from upland areas. In the picture above you are looking at hundreds of metres thickness of rock units that represent repetitive cycles of deposition from rivers, as the channels changed course cutting new channels through earlier deposits, flooding leaving sheets of fine mud which subsequently dried in the tropical sun, only for the cycle of events to be repeated. The picture below is an interpretation of this environment. A present day equivalent would be the rivers of the Northern Territory, Australia.
This was a significant time for the Earth, with the development and spread of primitive plants with root systems which would start to bind sediments and lead to the first soils. The plants began removing carbon dioxide from the atmosphere, which in the Devonian was at very high levels with very high global temperatures.
Limestones – calcretes
Not far from the top of the climb you come across the area shown below. Thin bands of knobbly pale grey limestone outcrop in the path. Known as the Ffynnon Limestone, it is a good example of limestone which has not been formed in water, but rather in the semi-arid flood plains, by precipitation from groundwater evaporating in the hot sun. Limestones formed in this way are called calcretes. They are common throughout the Devonian rocks deposited on the land. The calcretes here are thick enough and and extensive in outcrop to have been named and form a feature running around the upper slopes of the valleys cut into the rocks.
Above the Ffynnon Limestone as more beds of red-brown sandstones and mudstones known as the Senni Formation. Unlike the Freshwater West Formation which is around 60% mudstones the Senni is dominated by sandstones making it more erosion resistant character as displayed in the way it caps the flat tops of the Black Mountains. During the time the Senni was deposited coarser sediments would have been arriving on the flood plains, from probably due to uplift of the source rocks to the north.
Strange deformed rocks – soft sediment deformation
A little higher up from where the Ffynnon Limestones occurs the strange features shown in the next picture can be found. You will need to leave the path and carefully follow the rock outcrops around on the side overlooking the Olchon Valley.
These convolutions formed when the sediments were still in there soft state, not yet cemented (lithified). The deformation could result from slippage down a slight slope, or by the loss of water (dewatering) caused by shock from earthquakes.
In the same area as the soft sediment deformation exposures can be found evidence of the flowing water (fluvatile) environment in which this rock was initially deposited. Cross-bedding is indicative of sediment being deposited from a flowing medium – water or wind. In this case it was water, with erosion surfaces cutting into earlier deposits as the river channels change path.
Black Hill summit
When you reach the flat plateau of Black Hill you can either return back along the spine of the Cats’s Back or continue for a much longer walk around the top of the Olchon Valley to meet Offa’s Dyke path from which a number of paths descend into the Olchon Valley.
The narrow ridge of the Cat’s Back was probably the result of ice carving away the rock on either side, forming the narrow ridge, a feature known as an arête.
The image below is a reconstruction of how the area may have looked during the peak of the last glacial period – the Devensian – around 25,000 years ago. It is generally considered that glacial ice did not overtop the Black Mountains at this time, but probably did in earlier glacial events.
The hump in the western side of the Olchon Valley (see photo below) is probably a moraine (pile of debris dumped by ice) in a glacial event before the Devensian.
For more information on the Cat’s Back and surrounding area visit the following pages on the deeptime.voyage website. Deeptime voyage on the Cat’s Back. Field Trip to the Cat’s Back. This information along with maps and GPS guidance to find features like the soft sediment deformation can be found in the apps Voyager – Deeptime and GeoExplore both available on the App Store and Google Play.